A liquid crystal display device is an electrooptical device comprising a liquid crystal material whose optical characteristics vary according to an external electrical field. For example, in a TN (twisted nematic) type liquid crystal display device, an alignment layer is provided on the surface of a solid Substrate so that a liquid crystal molecule on the surface of the substrate is in a uniform alignment state to the surface, and the surface is subjected to a rubbing treatment. That is, a liquid crystal alignment layer has been obtained by preparing a thin film of a polymer such as polyimide, polyvinyl alcohol and polyamide on a solid substrate such as a glass plate according to spin-coating, printing, dipping and the like, curing it and rubbing it in one direction by means of a cloth or the like, and a liquid crystal molecule is arranged in one direction by said liquid crystal alignment layer. Since polyvinyl alcohol among these polymers is poor in heat resistance and moisture resistance, it is not used practically. Generally, polyimide and polyamide are practically used. It is because they satisfy the following conditions:
a) the alignment is chemically stable to heating which is carried out when two substrate are sealed face to face; PA1 b) showing neither solubility nor swelling characteristics to liquid crystal; PA1 c) having a high moisture resistance; and PA1 d) having a high layer-forming properties.
In addition, it has been known that a pretilt angle of a liquid crystal molecule can be obtained by subjecting these polymer films to a rubbing treatment.
Besides, a display device with super twisted birefringement effects (STN) which is excellent in display contents has been developed for a large-sized display (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984)). For super twisted birefringement effects is used in a nematic liquid crystal blended with a chiral agent, an optically active material. In it, a liquid crystal molecule twists at 180.degree. to 270.degree. in a liquid crystal display device while that of a TN type twists at 90.degree. or so, and the larger a twist angle, the better dependence upon visual angles. A large pretilt angle of liquid crystalline is required in order to enlarge a twist angle. Specifically, a pretilt angle is needed to be at least 5.degree. in order to obtain a twist angle of 240.degree. to 270.degree.. At present, a liquid crystal display device with a twist angle of 240.degree. to 270.degree. is made practically using a conventional polyimide alignment layer from the viewpoint of heat resistance and chemical resistance.
However, though a polyimide layer is generally applied first of all on a substrate as a polyamic acid solution and then heated for ring closure, the layer gets tinged during heating to mar the appearance of the device and the light resistance of the layer. Further, it is necessary to heat it at a temperature of at least 250.degree. C. in order to carry out a ring closure reaction, which may cause the deterioration of a plastic substrate and a colored filter. In addition, there is a problem that its adhesion with a glass substrate is not so strong.
Conventionally, as a means of enlarging a pretilt angle of a liquid crystal molecule are known a method of forming a oblique deposited layer of silicon oxide or the like and a method of forming a film of an organic polymer or the like having a long chain (Japanese Patent Public Disclosure Nos. 177514/89 and 297819/87) and subjecting the film to a rubbing treatment. However, there are problems that since oblique deposition of silicon oxide or the like is carried out according to a vacuum system, the cost for mass production becomes high and its producing ability is poor due to a batch treatment. Further, a method of subjecting a film of an organic polymer or the like having an alkyl long chain to a rubbing treatment has a problem that a pretilt angle changes largely according to a rubbing treatment and an annealing temperature (Japanese Patent Public Disclosure No. 197927/91).
On the other hand, Japanese Patent Public Disclosure No. 62616/89 discloses a random copolymer containing a polysiloxane compound and a polyaramide compound as an alignment layer material in which a pretilt angle has little dependence upon an annealing temperature. However, the alignment layer is characterized in that its pretilt angle is small; since a high pretilt angle is demanded in STN and TFT-LCD needing a broad visual angle, it is not suitable for practical use because its pretilt angle is smaller.